This invention is generally directed to developer compositions, and more specifically, the present invention relates to developer compositions containing carriers. In embodiments of the present invention the carrier particles are comprised of a mixture of a first carrier comprised, for example, of an iron core, such as an iron powder core available from Hoeganaes Corporation as an “atomized steel powder”, and referred to as a “steel core”, especially a rough core, that is a core with a nonsmooth surface, and a second carrier comprised of a spherical magnetite core and embodiments wherein each of the cores can be of a similar size diameter. The second carrier core in amounts, for example, of from about 10 to about 30 weight percent, primarily functions to increase the developed mass per unit area (DMA) of a solid image by up to about 80 percent, and more specifically, from about 30 to about 65 percent compared to the use of a rough steel core, permits an excellent optical density increase of the obtained solid image by approximately the same factor as the DMA developability characteristics of developers within which it is contained, and permits improvements development stability in low throughput aging (for example, prolonged such as for 100 cycles, and more specifically, from about 100 to about 1,000 cycles at a low, about for example 5 percent, area coverage corresponding to a full page of text). The first steel cores rough morphology primarily maintains the conductivity of the developer at, for example, from about 10−8 to about 10−7 (ohm-cm)−1 as measured by known methods, such as determined in a magnetic brush cell at 10 volts. Conductivity of a developer of carrier and toner in a donor roll development system is preferably maintained at least from about 10−9 (ohm-cm)−1 to about 10−4 (ohm-cm)−1 to provide sufficient toner reload on the donor roll. Deficient reload appears upon printing large solid area images as fading of the solid or half-tone area after one full revolution of the donor roll. Typically, an increase of lightness (L*) by 3 or less is permitted in reloaded solid area. Changes of L* by greater than 3 are visible to a naked eye, and thus constitute a print defect. More specifically, in embodiments the carrier particles can be generated from a mixture of a spherical carrier and a rough carrier, the spherical carrier being comprised of a magnetite core and a rough carrier comprised of a steel core as illustrated herein, inclusive of mixtures of coated spherical and coated rough carriers, and note that the rough carrier is conductive primarily due to its rough morphology. The pure carriers in embodiments can possess close conductivities of, for example, about 10−8 to about 10−7 for rough steel and from about 10−9 to about 10−8 for magnetite. However, developer conductivity falls with TC much faster with the spherical carrier than with a rough carrier core. For example, the conductivity of a developer comprising a rough steel or spherical magnetite carrier with an average particle size of 65 micrometers and toner with an average particle size of 5.6 micrometers at a toner concentration (TC) of 0 percent (carrier only), 1 percent and 4.5 percent. With rough carrier, developer conductivity was 9×10−8 (ohm-cm)−1, 1.1×10−8 (ohm-cm)−1 and 3.2×10−12 (ohm-cm)−1, respectively; when the developer was comprised of a spherical carrier, developer conductivity was 3.4×10−8 (ohm-cm)−1, 4.9×10−10 (ohm-cm)−1 and 1×10−14 (ohm-cm)−1, respectively. Thus, for example, there can be provided in accordance with aspects of the present invention developers with a conductivity of from about 10−6 to about 10−15 (ohm-cm)−1, and yet more specifically, wherein the developers can possess semiconductive or moderately conductive such as from about 10−8 to about 10−11 (ohm-cm)−1 characteristics, that is wherein the conductivity of the carrier particles are in between conductive and insulative carriers, and more specifically, wherein semiconductive refers, for example, to a carrier with a conductivity of from about 10−9 to about 10−13 (ohm-cm)−1. The carriers of the present invention may be mixed with a toner of resin, colorant, and optional toner additives, and more specifically, toners generated by known emulsion/aggregation processes, and wherein the toner volume average diameter can vary, for example; the diameter can be from about 2 to about 25 microns, and more specifically, from about 4 to about 7 microns, and yet more specifically, from about 5 to about 6 microns, to provide developers that can be selected for the development of images in electrostatographic, especially xerographic, imaging systems, printing processes, digital systems, more specifically hybrid development, reference for example, U.S. Pat. No. 5,032,872, the disclosure of which is totally incorporated herein by reference.
Examples of carriers in embodiments of the present invention include those comprised of a suitable carrier core as illustrated herein, and a polymer thereover, such as polymethylmethacrylate (PMMA), polyvinylidenefluoride, polyethylene, copolyethylene vinylacetate, copolyvinylidenefluoride tetrafluoroethylene, polystyrene, polytetrafluoroethylene, polyvinylchloride, polyvinylfluoride, polylbutylacrylate, copolybutylacrylate methacrylate, polytrifluoroethylmethacrylate, polyurethanes, and mixtures thereof, especially a mixture of two polymers. Conductive carrier mixtures can also include a suitable carrier core, a polymer thereover and a conductive component, such as a conductive carbon black dispersed in the polymer coating.
Advantages of the carrier mixtures of the present invention in embodiments include controlling and preselecting the triboelectric charge and conductivity of the carrier, the formation of homogenous mixtures, excellent carrier coating adherence, stable charging characteristics, carrier design flexibility and freedom, economical carrier formation, increased developability, development stability to low throughput aging, developer conductivity at high levels of about 10−10 to about 10−8, excellent stable charging characteristics, and the like.